1. Field of the Invention
This invention relates to silver halide elements that contain an arylhydrazine compound useful in producing high contrast images. More particularly, this invention relates to ultrahigh contrast silver halide elements which can be handled in bright light room conditions and processed at a rapid rate.
2. Description of Related Art
Most conventional photographic materials are sensitive to visible light and, accordingly, must be handled under low intensity red, yellow, or amber lighting, commonly referred to as "darkroom conditions", to prevent undesired exposure of the material. Also known in the art, however, is a type of photographic material which can be handled under bright room conditions for a limited period of time without detrimental effects on the material. Such photographic materials are variously referred to in the art as "bright light films," "white light films," "daylight films," or "room light films".
Photographic materials which can be handled and processed in a bright room are specifically those which can be used in a room having illumination of at least 200 lux from a fluorescent lamp having reduced ultraviolet ray emission, or a filtered light not having the wavelength below about 420 nanometers (nm) as a safelight source. Typically, these photographic materials can be safely handled in bright room conditions on the order of 25 to 60 minutes, without detrimental effects on the film, such as fogging. To obtain such results, these photographic materials have greatly reduced sensitivity to visible light, that is about 1/1,000 to 1/10,000 that of ordinary darkroom photographic materials.
Exposing these photographic materials requires the use of a high intensity light source rich in ultraviolet light, such as, for example a high pressure mercury lamp, a metal halide lamp, a microwave discharge type mercury non-electrode light source, or a xenon lamp. To produce the high intensity required to expose bright light films, the above sources are broader than the point light sources used for conventional darkroom films and consequently emit more diffuse light. In order to obtain sharp character images, line images or dot images, by contact exposure of such bright light silver halide photographic material with a broad high intensity source, the photographic material is required to have a high contrast photographic characteristic, i.e., a contrast value of 10 or more. As is well known in the art, contact exposure is when a film is in contact with a negative during exposure. Various image forming systems for such materials have heretofore been provided.
To obtain the desired high contrast, the emulsion for the photographic material typically contains silver halide grains having a high proportion of silver chloride, which have been chemically sensitized by sulfur, noble metal, or reduction sensitizers or combinations thereof. Sulfur is the most common chemical sensitizer used. However, chemical sensitization, particularly sulfur sensitization, extends the intrinsic light absorption of the photographic emulsion to light wavelengths longer than 420 nm, reducing the room light tolerance of the photographic material.
To compensate for this effect, yellow dyes having a peak absorption in the range of 400 to 550 nm are typically added to the photographic emulsion layer and/or to a layer above the emulsion to reduce sensitivity to light above 420 nm wavelength. However, dyes added for the purpose of enhancing the safelight tolerance of these photographic materials often reduce the contrast enhancement arising from chemical sensitization.
Recently, the use of the lower wattage metal halide light sources and quartz iodide light sources has become increasingly popular because of decreased cost, energy savings and improved convenience of use. These lower wattage light sources, however, have reduced ultraviolet light emission, which means that the bright light films must have a higher sensitivity for use with these light sources. In order to provide typical desired contact exposure times of about 5 to 15 seconds, photographic materials designed for use with these lower wattage light sources require about an order of magnitude higher ultraviolet sensitivity compared to those designed for use with other high intensity light sources.
Furthermore, it is desirable to operate these lower energy light sources in a bright room, preferably in a room where the illumination intensity is fully equivalent to the typical office environment (about 540 lux). A method for preparing photographic silver halide emulsions having high tolerance to such room light conditions is described in U.S. Pat. No. 5,252,449. According to this method, rhodium-containing high chloride emulsions are sensitized by black and white developing agents. In the examples, mid-gradients, identified as contrast values, are limited to about 10.
There are applications, however, such as reverse imaging of very fine characters or the reproduction of images having soft edges, where ultrahigh contrast emulsions are needed to obtain high contrast images. "Ultrahigh contrast" in photographic silver halide elements is defined here as silver halide emulsions providing a contrast value above 15. Although litho development has been used to obtain ultrahigh contrast, development times are long, typically in excess of 1 minute, and the stability of the developer is limited to a few days. Aerial oxidation of the developer which gives rise to the stability problems is caused by the low sulfite content of these developers. Sulfite is generally added as a preservative to developers. Sulfite is low in litho developers because it interferes with the infectious litho development mechanism. To avoid these disadvantages, emulsions containing hydrazine derivatives have been used to produce ultrahigh contrast with developers high in sulfite content. The stability of the developer against aerial oxidation is greatly improved and development times are also reduced, generally to 30 to 45 seconds. However, to obtain rapid development, the pH of these developers has generally been high, often in excess of 11.6.
Methods for obtaining ultrahigh contrast in photographic elements at lower pH, e.g., pH of 10.8, by the use of particular aryl hydrazines described in U.S. Pat. Nos. 4,937,160, 4,939,067, 5,013,844, and 5,190,847, provide advantages in replenishment rates, disposal of exhausted solutions, and corrosion resistance of the development apparatus over the prior art. However, preferred emulsions for these inventions contain less than 50% chloride and the examples use emulsion containing either rhodium-free bromide or rhodium-free iodobromide grains, (2% mole iodide) developed in the range of 30 to 45 seconds. The application of photographic elements containing the aryl hydrazine is for the preparation of half tone images from continuous tone images, the reproduction of line images and photomasks for printed circuits or other products of photofabrication, and preparation of printing masters by phototypesetting.
The requirements for hydrazine technology are quite different, however, when used to produce ultrahigh contrast photographic silver halide elements designed for contact use under bright room conditions. As mentioned previously, the photographic elements used for these applications contain high chloride, preferably all chloride, grains which are desensitized with rhodium or iridium or other inorganic or organic agents. The desensitizing agents trap photoelectrons during exposure, reducing photographic sensitivity. However, they also can receive electrons donated by the hydrazine derivative during development, inhibiting nucleation of infectious development. While this effect can sometimes be overcome by adding relatively high levels of hydrazine derivative compounds, high levels of derivative compounds degrade safelight tolerance of these films, as well as coatability, sensitometric aging, and certain physical properties. Hence, very effective hydrazine derivatives are needed for room light contact films so that ultrahigh contrast can be obtained with acceptable amounts of the hydrazine compound. Also, it is imperative that any sulfur which may be part of the hydrazine derivative used in these emulsions be structurally bound in such a way that it does not become labile during the sensitization or development processes, resulting in the aforementioned reduction in safelight tolerance associated with sulfur sensitization.
Furthermore, the hydrazine derivatives for room light contact films need to be extremely effective nucleating agents to meet the rapid processing requirements for these systems, where the rapid access development time of at least 20 to 35 seconds is an industry expectation. The development time of hydrazine-containing systems can be accelerated by high pH development, i.e., pH of the developer around 11.6. But there are numerous aforementioned disadvantages of development at such high pH. U.S. Pat. No. 5,384,232 describes development accelerators, such as 1-phenethyl-2-picolinium bromide (PPB) and related derivatives which accelerate development of hydrazine-containing systems. However, the emulsions in the examples of pending application Ser. No. 08/040,247 contain 80% or less chloride and very little or no rhodium or other inorganic or organic desensitizers required for room light films.